Figure. 12-lead ECG and rhythm strip recorded from an older man with shortness of breath.

Clinical Scenario: The ECG in the Figure was interpreted as showing sinus tachycardia with non-specific ST-T wave abnormalities. Do you agree?

Interpretation/Answer: The rhythm is regular at a rate of about 140 beats/minute. The QRS complex is narrow. Upright P waves appear to precede each QRS complex with a fixed PR interval in lead II. Alas, this is not what is really occurring.

The most commonly overlooked sustained cardiac arrhythmia in our experience is atrial flutter. The 12-lead ECG shown here illustrates why. There certainly does appear to be an upright P wave preceding each QRS complex in each of the inferior leads in this tracing. However, this peaked upright deflection that precedes each QRS deflection is only one of two flutter waves that are seen to occur between each R-R interval. The second flutter wave is hidden and very easy to overlook unless carefully searched for. In fact, T waves are subtly notched in several leads (intermittently in each of the inferior leads, as well as in leads V1 and V2). Use of calipers allows one to precisely march out regular atrial flutter activity at twice the ventricular rate (ie, at 280/minute).

The easiest way to avoid overlooking the diagnosis of atrial flutter is to maintain a high index of suspicion for this arrhythmia. Practically speaking, the differential diagnosis of a regular SVT (supraventricular tachycardia) will most often consist of 3 entities: i)sinus tachycardia; ii) PSVT (paroxysmal supraeventricular tachycardia; and iii) atrial flutter. Because the atrial rate of untreated flutter in adults is almost always close to 300/minute and the AV conduction response is most commonly with a 2:1 ratio, any regular SVT at a rate between 140-160/minute should be thought of as a possibly due to atrial flutter until proven otherwise. Subtle notching in the T waves of several leads in this tracing is suggestive of this diagnosis. Equally suggestive is the unusual sharpness of inferior T waves and the downward deflection that follows the T wave in leads aVR and aVL. Application of a vagal maneuver may prove to be diagnostic (as it was here) by transiently slowing the AV response enough to allow clear visualization of underlying atrial
flutter activity at a regular rate of 280/minute.